1. Field of the Invention
This invention relates to apparatus for determining the concentration of toner particles in electrographic developer mixtures. More specifically, this invention relates to toner concentration monitoring apparatus of the type wherein toner concentration is determined by sensing the reflectivity of the developer mixture. Fresh toner is added when needed to maintain the toner concentration at a desired level.
2. Description of the Prior Art
In the electrographic reproduction process, the surface of a radiation-sensitive photoconductive member which may comprise a layer of photoconductive material disposed on a conductive backing, is given a uniform electrostatic charge and is then image-wise exposed to a pattern of actinic radiation corresponding to the indicia on a document or the like being reproduced. Such exposure serves to selectively dissipate the uniform charge on the surface, leaving behind a latent electrostatic image which can then be developed by contacting it with an electrographic developer mixture.
In general, electrographic developer mixtures comprise a mixture of suitably pigmented or dyed resin-based electroscopic particles, known as toner, and a granular carrier material which functions to carry such toner and generate a triboelectric charge on such toner. The development of the latent electrostatic image occurs when the developer mixture is brought into contact with the electrostatic image-bearing surface. Such contact is commonly effected by either cascading the mixture over such surface or brushing the surface with one or more rotating magnetic development brushes, the "bristles" of which comprise chain-like arrays of toner-carrying carrier particles. Upon contacting the electrostatic image-bearing surface, the toner particles, being charged to a polarity opposite that of the electrostatic image, are separated from the carrier particles and are selectively deposited in an imagewise configuration on the image-bearing surface to form a developed image. The developed image is thereafter transferred to a paper receiving sheet and fixed thereto by any suitable means, such as heat or pressure, to form a copy of the original document. As successive toner images are formed, toner particles are depleted from the developer mixture, requiring subsequent replenishment to avoid a gradual deterioration of image quality, notably a reduction in image density.
To avoid the necessity of periodic manual replenishment and the operating difficulties often encountered as a result of over or under replenishment, the electrograhic apparatus frequently includes means for metering additional toner into the developer mixture. For example, an interface mechanism is provided for receiving a replacement toner container upon removal of an exhausted toner container. A replenishment mechanism, operatively associated with the interface mechanism, dispenses toner from a received container in response to a signal from a toner concentration monitor. Exemplary of photoelectric or optical toner concentration monitoring devices is the device disclosed in U.S. Pat. No. 3,233,781 issued to W. J. Grubbs which utilizes the reflectivity of a developer mixture having toner and carrier particles as a means for monitoring the concentration of toner particles in the developer mixture. Toner particles, usually being black, possess light absorbing surfaces and reflect less radiant energy than the carrier particles. Thus, the reflectivity of the developer mixture depends upon the relative proportions of the mixed particles. According to the Grubbs disclosure, the reflectance of the developer mixture is monitored by directing light from an incandescent lamp toward the developer mixture and detecting the light reflected by the developer mixture with a photocell. This photocell along with a similar photocell which is illuminated directly by the lamp are employed as variable resistance arms of a bridge circuit. The bridge circuit activates a toner replenishing device in response to a predetermined change in the ratio of photocell outputs, such change being characterized by an unbalance in the circuit.
A significant improvement is disclosed in commonly assigned U.S. Pat. No. 3,876,106 to Rowell et al issued Apr. 8, 1975. In the Rowell apparatus, a light emitting diode (LED) illuminates a developer mixture of toner and carrier particles. The apparatus includes first and second photosensitive means, the first being disposed to receive LED radiation reflected from the developer mixture and the second being disposed to receive radiation directly from the LED. The first and second photosensitive means produce first and second analog signals representative of the reflectivity of the developer mixture and the intensity of the radiation of the LED, respectively. The first and second analog signals are then converted to digital signals. This apparatus further includes a programmable digital computer with a stored program, which in response to such first and second digital signals produces, in accordance with such stored program, a representation of the toner concentration of the developer mixture. In response to input signals from the photosensitive means, the apparatus from time to time adds fresh toner to the developer mixture to maintain a substantially constant developer mixture reflectance.
As pointed out in the above-referenced patent application to Reid et al, it has been observed that with usage of a developer mixture, the reflectivity of the developer mixture is not always a reliable indicator of toner concentrator in the developer mixture. A portion of the change in developer mixture reflectivity is directly attributable to carrier particle scumming. Such scumming refers to the fact that as the developer mixture is used, the mechanical friction between the toner and carrier particles causes a thin film of toner material, which is not released from the carrier particles for development, to be coated on the outer surface of the carrier particles. As carrier particle scumming increases, the reflectivity of the carrier particles decreases, and consequently, the reflectivity of the developer mixture decreases below that reflectivity level which would have been obtained had there not been carrier particle scumming.